Understanding the split personality of Iceland’s volcanoes

Not sure why a few plumes of molten rock could somehow spread ash across …

The initial images of the Eyjafjallajökull eruption showed the sort of dramatic spires of molten rock that we associate with Hawaiian volcanoes. The next time it made the news, it was because air travel throughout Northern Europe had been shut down as a huge cloud of ash spread slowly across the UK and Scandinavia—very un-Hawaiian. To get a better sense of why this Icelandic volcano was showing such a split personality, we got in touch with the American Geophysical Union, which handed us on to Dr. Jeff Karson, who's chair of the Earth Sciences department at Syracuse University. Dr. Karson patiently explained what makes volcanism in Iceland distinct.

A look down on the Eyjafjallajökull eruption as it burns through the local glacier.Credit: NASA

If you're like me, and know just enough Geology to be dangerous, you'd probably divide volcanoes into two categories: hotspots like Hawaii, where molten rock pours out as gently as anything that's 1,200°C possibly can, and volcanism associated with subduction zones, which tends to produce massive, explosive eruptions, such as the ones at St. Helens and Pinatubo. There is, of course, a third kind, the eruptions associated with the spreading of mid-ocean ridges. But these generally take place deep underwater, and are rarely captured on film.

Iceland is distinct because it's the product of a huge hot spot located directly under the Mid-Atlantic ridge. It's not unique in that regard; Karson said that the Azores and Galapagos Islands are the product of similar situations. But Iceland is much hotter and more active than the others.

If it's not on a subduction zone, however, you might not expect it to produce the sorts of explosive eruptions that send clouds of ash across large areas of the North Atlantic. Karson said there are two factors that can make Icelandic volcanoes pack a punch. The first is that, in addition to the basaltic magma associated with mid-ocean ridges, Iceland's volcanoes produce significant amounts of rhyolite, which is silica-rich and, more significantly, contains a lot more volatile substances. As the rhyolitic magma reaches the surface and pressure is released, the result can be an explosive eruption. As Karson put it, the cause is different from the explosive volcanic eruptions that occur at subduction zones, but the result can look very similar.

Individual magma chambers beneath Iceland can have varying mixes of basaltic and rhyolitic materials, which means that individual volcanoes on the island may have a complicated eruption history.

The other factor that adds to the explosiveness of Iceland's volcanoes is the ice itself. When magma at 1,200°C comes in contact with water or ice, it vaporizes it instantly, which produces the same sorts of high-pressure gasses that drive the explosiveness of rhyolitic eruptions. (Karson's research apparently involves mixing melted rock and ice in outdoor spaces on the Syracuse campus—he joked that it's the most interesting geology that upstate New York has experienced since the last glaciation.)

The plume of ash heads across the Atlantic, on its way to annoy travelers throughout Europe.Credit: NASA

So, although the Eyjafjallajökull eruption has generally involved basaltic magma, its interactions with the local ice and water has given it the sort of explosive punch that can send a plume of ash high enough to spread across parts of Europe. Still, for now at least, Karson says there's no reason to suspect that we'll see anything more than regional effects. Eyjafjallajökull simply doesn't produce eruptions as powerful as subduction zone volcanoes such as Pinatubo, which can send aerosols into the stratosphere and cool the climate. Still, the eruptions can last months to years, so Northern Europe may need to prepare for an extended period of disruptions.

But there is a caveat—Karson noted that, historically, activity at Eyjafjallajökull has frequently preceded eruptions at the nearby Katla, which is a much bigger volcano and contains more rhyolitic magma. Karson said that there's evidence that Katla has produced events that covered all of Iceland in ash, and could have what he termed "broad regional to global impact." Signs of activity there would be a major source of concern—especially for Iceland.

As the chat wrapped up, Karson wanted to make sure that the eruptions and disruption were kept in perspective. With the recent spate of major earthquakes, some people have been wondering whether the planet is in an especially active state. It's not. These things go on all the time, Karson said, but they're often under water or in distant regions where the impact goes unnoticed. The difference is mostly that recent events have taken place where people are more aware of it. "We take for granted that air travel just happens," Karson said. "The earth is an active planet, and we need to work around things like this."

In any case, if you're curious, a helpful person has provided an audio clip of the proper pronunciation of Eyjafjallajökull. The ESA has also produced an animation of the spread of the ash plume earlier this week.